webcodecs: Handle exceptions happening during VideoEncoderSupport conversion

There should be no exceptions while copying VideoEncoderSupport because
all dictionary values should already be checked for range by v8.
But fuzzer doesn't check ranges, so outlandish numbers can get into VideoEncoder
while fuzzing. This change takes care of it.

Bug: 1231597
Change-Id: Ibce8f6da1e5f199fdd8cabe0ebf3e2bc9772f594
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3057637


Reviewed-by: Dan Sanders <sandersd@chromium.org>
Commit-Queue: Eugene Zemtsov <eugene@chromium.org>
Cr-Commit-Position: refs/heads/master@{#906342}